Mega Evolutions.
For one battle, a Pokémon breaks its own limits. These 98 Mega and Primal forms each rewrite typing, stats, and abilities, and with Legends: Z-A they are back at the centre of competitive play. Every one has a dedicated page.
Mega Evolutions
A held Mega Stone and a Key Stone push a Pokémon past its normal ceiling for the length of a battle. Mega Evolution debuted in X and Y and returns as a centrepiece of Pokémon Legends: Z-A. A handful of species (Charizard, Mewtwo) carry two distinct Megas.
Primal Reversions
Kyogre and Groudon tap the Blue and Red Orbs to reach Primal forms, the ancient states tied to the weather trio. Both gain enormous bulk and offence in Omega Ruby and Alpha Sapphire.